Backstory:

This department of the Black Mesa Research Facility was evacuated
after the catastrophic resonance cascade in the main facility.
Even though, the team in the Sigma-facility has managed to create a stable Portal to the Xen- Dimension,
which points into an underground cave of Xen and is therefore undiscovered by the Combine.
While originally planning to re-establish the research in the facility, humanity suffered defeat in the war against the Combine forces.
It is therefore not known, if anyone in charge of the facility has survived of if it is abandoned and forgotten.

What will Black Mesa Sigma become for you now?
An opportunity to attack the combine where they don´t expect it?
A threat to everyone that must be destroyed?

A safe place to seek refuge from a world falling apart?
…You decide.
But keep in mind that the Combine may already be on their way…